Many vehicles have hydraulic brake and/or steering systems which are powered by an engine-driven hydraulic pump. Some such vehicles are so heavy that they cannot be manually steered or braked in accordance with legal requirements in the case of engine power failure. Therefore, an auxiliary source of fluid pressure has been required in these cases to provide hydraulic pressure to the steering and/or brake systems. This need for an auxiliary source of fluid pressure has been satisfied in the past by different means. For example, accumulator systems are described in U.S. Pat. No. 4,326,558, issued Apr. 27, 1982 to Gage and U.S. Pat. No. 4,303,089, issued Dec. 1, 1981 to Gage et al. Systems with electrically powered auxiliary pumps are described in U.S. Pat. No. 3,820,620, issued Jun. 28, 1974 to Miller et al. and in U.S. Pat. No. 3,995,711, issued Dec. 7, 1976 to Kittle et al. Systems with ground-driven auxiliary pumps are described in U.S. Pat. No. 4,160,490, issued Jul. 10, 1979 to Bexten et al., in U.S. Pat. No. 4,076,096, issued Feb. 28, 1978 to Hushower et al. and in U.S. Pat. No. 4,015,681, issued Apr. 5, 1977 to Shore et al. The typical ground-driven auxiliary pump system includes a fixed displacement ground-driven pump, a series of check valves to provide bi-directional operation, and an unloading valve to reduce heat generation when the engine-driven pump is operating. Historically, accumulator and electric powered systems have been preferred over ground-driven systems because considerations of cost, complexity and power loss problems associated with ground-driven systems have made them impractical, even though performance and reliability may be superior. It would be desirable to have a simple, low cost and efficient ground-driven auxiliary pump system.